Controling Common Garden Pests & Diseases

Common garden pests are snails, grubs, aphids and grasshoppers. Ensuring that plants are watered, are in a suitable climate, healthy and have enough food will keep away the pests. Insects are attracted to sick or weak plants. Sprays are not a necessity for the garden. It is possible to prevent the use of sprays and control the problem by hand, by removing damaged and pest affected materials, diagnosing and confronting the problem as soon as it is noticed. If this does not have a positive affect then sprays may have to be used. Chemicals should not be used unless the pest is identified and should be used correctly, under the guidance of the instructions.

Fungi, bacteria and viruses are common plant diseases. These diseases may be spread by insects, spores, soil or debris. Gardening tools can also carry disease from one plant to another. Reducing the spread of disease in plants cab best be achieved by cleaning pots and gardening tools after each use. The removal of the diseased parts of the plant is the best way to combat the problem.

African Black Beetles

Commonly these beetles attack lawns, vegetables and ornamental plants. In summer the beetles and their grubs feed on the roots of the lawn causing large dead patches. The adult beetles are most active in mating time, which occurs in early spring and late summer.

Control: Infested areas should only be treated with chemicals if necessary. Sprinkling fenamiphos granules over the lawn and watering well can also control this problem.

Aphids (Greenfly)

These insects are commonly green, although sometimes pink, black or grey. They feed in large groups, sucking the sap from buds and new shoot tips causing them to die and distort the flowers and can spread plant diseases. Aphids sometimes will feed on leaves and roots. Aphids festations increase in spring and summer.

Control: Aphids are easily controlled by squashing the clusters by hand or squirting them with a strong jet of water. More effective is soapy water mixed with white oil, pyrethrum or fatty acid based sprays all non-toxic and safe to use.


Borers often attack old, weakened or damaged shrubs and trees, entering the effected area. They tunnel into the plant leaving a mass of sawdust.

Control: Best defense is to keep plants well fed and watered. Scrape away damaged bark and attempt to bring out the borers. More effective is to squirt a few drops of methylated spirits or kerosene into the borer tunnels and putty up the holes to prevent water entering.

Bronze Orange Bugs

These bugs are serious pests of citrus trees. The bugs suck sap and cause shoots to wilt and die, and the flowers and fruit to fall.

Control: Chemical sprays such as dimethoate or omethoate are effective. Although dimethoate should not be used on kumquats, Seville orange trees and Meyer lemons. It is best to spray in the early spring when the bugs are still juveniles. Goggles should be worn when spraying as these bugs can squirt a smelly, acidic liquid.

Cabbage White Butterflies

 These caterpillars are destructive to all species of the cabbage plant. The caterpillars feed from under the leaf starting from the outside. Infestations are worse in late summer and early autumn.

Control: Spray with carbaryl or with bacillus thuringiensis (a disease that affects only caterpillars) or use a contact powder such as derris or cabbage dust.

Fruit Flies

Attack many fruits and vegetables, they lay eggs into the fruit as it ripens and hatch into maggots. As the maggots mature they drop to the ground where they soon emerge as adult flies. This cycle can take approximately 5 weeks. Fruit flys attack in spring and continue through summer.

Control: Control is essential. Control is almost impossible without spraying, which should be done fortnightly from fruit set. If fruit falls, do not leave it on the ground for more than 3 days; and do not bury the fruit.

Leaf Miners

Leaf miners are the larvae of various moths, flies, wasps and beetles which tunnel inside the leaves of plants. Some species attack only a specific plant while others attack many.

Control: Remove infected leaves, if damage is minor. If spraying is necessary, use a penetrant spray or systemic as the larvae are protected by the leaf surface.

Mealy Bug

Mealy bugs are say-sucking insects which can cause wilting of young shoots. They attack indoor plants, especially cactus, ferns, palms, vegetables and succulents.

Control: Natural predators are ladybirds, wasps and small birds. Although chemical control is usually a necessity as they gather in crevices and beneath the soil. Systemic insecticide such as omethoate or immerse potted plants in a maldison solution can be used.


Mites attack a large variety of plants, often indoor. They suck sap from the leaves and attack increasingly in the dry, hot weather.

Control: Natural predators include ladybirds and the predatory mite, which can be released onto the affected area and will discard a toxic spray to kill the mites. Sprays such as miticides can be used, although are not always effective.


Sawfly larvae feed on the foliage of a wide variety of plants, which they may defoliate. Control: Natural predator is the ladybird. Removing the twigs and squashing them or use maldison spray.


Small immobile bumps, which attack many types of plants by sucking sap. Severe infestations can cause weakening and death of part of the plant.

Control: If small in number they can be washed off with soapy water and a brush. Larger numbers can be sprayed with white oil and maldison until all the insects have been killed.

Snails and Slugs

Numbers can increase rapidly if not controlled and are most active during spring and autumn.

Control: Keep areas where they are usually found clean, such as the rims of pots, dense leaves close to the ground and under bricks. Sawdust, coarse sand or finely crushed stone can be used to deter the snails and slugs. Snail baits are effective although they attract and are poisonous to dogs, so they should be scattered thinly.


Thrips are tiny, slender and black, light brown or cream in colour. They attack the leaves and flowers of a large variety of vegetables and ornamental plants, leaving a streaky or blasted appearance. The eggs are laid within the plant tissue and multiply rapidly.

Control: Often difficult to control as they are protected inside flowers or under the leaves. Sprays such as dimethoate and maldison control thrips. A second spray is recommended after 10 days.

White Curl Grubs

These grubs are the larvae of scarab or cockchafer beetles. They are fleshy white with a brown-orange colour head. They attack a wide variety of plants, feeding on the root systems causing significant damage to container grown plants.

Control: In small numbers they are controlled by natural predators and cause minimal damage. Controlling the adult beetle is best, which can be done by collecting and destroying them once they dislodge from the plants. Container plants affected should be repotted once the grubs are cleaned from the soil.


These are tiny flies which infest the underside of foliage of many plants, cuasing them to wilt and turn yellow.

Control: Parasitic wasps often keep the numbers down, although spraying with white oil or dimethoate every fortnight may be necessary.

Plant Diseases

Black Spot

A fungus disease which affects the leaves in plants, including roses, apples, pears, plums and quinces. Black spot causes the leaves to wither and fall prematurely. Black spot is worst when humidity id high or in tropical and subtropical regions.

Control: Cut off and dispose of infected leaves. Spraying with a fungicide such as captan, copper oxychloride and mancozeb may be necessary.

Collar Rot of Citrus

Occurs in citrus trees, causing the bark of the trunk close to ground level to flake and the wood to rot. Trees extensively affected will not produce new growth and will eventually die. Many citrus trees are grafted on disease-resistant rootstocks; when planted the graft should be well clear of organic materials.

Control: Cut away damaged bark to expose healthy and clean wood. Paint with copper oxychloride to cover the wound.

Downy Mildews

Downy mildews are a fungi which attack the leaves of vegetable, fruit and ornamental plants. They cause small yellow, pale green or brownish spots which then dry out and kill the leaf. In humid conditions grey patches will appear under each spot.

Control: Remove and discard of affected leaves. Zineb fungal spray should be used and sprayed also on the underside.

Infectious Variegation of Camellias

The cause of yellow marks on the foliage of camellias is believed to be a virus. Not all leaves will be affected and the plant will still produce flowers.

Control: Effected leaves should be removed. Diseased plants should be moved to prevent transmission to other plants and should not be used for grafting.

Petal Blight of Azaleas

A fungus which affects the flowers leaving brown or transparent spots. The flower softens as the spots enlarge. Prematurely the flowers shrivel and become papery, turning brown in colour. This disease spreads rapidly and favours high humidity and rainy weather.

Control: Bayleton azalea fungicide should be sprayed fortnightly on the plants, from the time the flower buds begin to show colour and until the bloom is completed.

Powdery Mildew

A group of fungi that coat leaves, fruits, flowers and young shoots with a white or pale grey ash-like film. Powdery mildew is spread rapidly from plant to plant by the wind.

Control: Remove affected foliage. Avoid overcrowding of these varieties of plants to avoid spreading. Spray with wettable sulphur or coat with sulphur dust at first sign of infection.

Root Rot

Root rot is caused by various fungi diseases. One of the most serious is cinnamon fungus. In hot weather a healthy plant may partially or completely wilt and die within a few days. Armillaria root rot is spread through the soil by flat, black cords. Trees infected lose vigour, the leaves turn yellow and the branches die. At the base of the dead trees of from the foots remaining in the ground clusters of golden toadstools appear.

Control: Remove the dead plant including the roots. Drench the soil with Fongarid or Ridomil. Early application of these can save a mildly affected plant.

Rose Canker

Caused by a fungus which enters through pruning cuts or wounds. Symptoms are dark reddish lesions and cracking bark of rose canes. Canker can cut off nutrients and water to growth further up by encircling the stem.

Control: Prune off and burn affected canes. Clean secateurs after use to prevent infection from plant to plant. Just above a healthy outward facing bud make a neat slanting cut.


Appear mainly on the upper surface of leaves and recognized as small yellow or orange spots. Powdery, raised pustules appear on the underside of the leave under each spot. Rusts can cause leaf fall and weakening of plants. Most attack a specific plant or a small group of related varieties and will not transfer from one host to another. However there are varieties for a wide range of plants.

Control: Treatment differ as different rusts affect many plants. Effective against most is zineb, sulphur or oxycarboxin. Clear away leaves as they fall to prevent possible carry-over of the rusts.

Sooty Mould

Dark fungal growth which forms on the sticky secretions (called honeydew) produced by sap-sucking insects, causing black mould to appear on leaves and stems, although this does not harm the plant directly.

Control: By removing the primary infection, such as the aphids or scale the sooty mould will disappear. Hosing with a strong spray will assist to clean up.